Method of separating aromatic hydrocarbons from mixtures containing them



March 4. 1969 REM ET AL 3,431,199

METHOD OF SEPARATING AROMATIC HYDROCARBONS FROM MIXTURES CONTAINING THEM Filed Jan. 20, 1967 26 b 31 12 31 m M M M T; 11 1 United States Patent US. Cl. 208-325 Int. Cl. Clog 21/12 Milan, Italy, assignors to S.p.A., Milan, Italy 20, 1967, Ser. No. 610,535 application Italy, Jan. 26, 1966,

17 Claims ABSTRACT OF THE DISCLOSURE Method of separating aromatic hydrocarbons from liquid hydrocarbon mixtures by charging the mixture to a multi-stage extractor at high temperature with a high boiling organic selective solvent, collecting a liquid extract phase with a high aromatic hydrocarbon content and a raflinate liquid phase essentially comprising nonaromatic hydrocarbons, conveying the extract after cooling to a decanter in which two distinct liquid phases are separated, recycling the heavier liquid phase to the extractor, conveying the lighter liquid phase to an azeotropic distillation column to which acetone vapor is admitted, and after fractionation, conveying as a reflux to the bottom of the extractor the lighter fraction of the non-aromatic rafiinate together with any aromatic hydrocarbons present in the ralfinate.

This invention relates to a matic hydrocarbons in a taining them.

A number of extraction and extractive distillation methods are known in the art for separating aromatic hydrocarbons from liquid hydrocarbon mixtures by means of selective solvents. The liquid-liquid extraction technique is generally adopted in combination with the extractive distillation technique, the latter being applied to the solvent loaded with aromatic hydrocarbons.

Azeotropic distillation methods are also known in oommercial practice, by which the non-aromatic constituents of the mixture are distilled by means of a stripping agent.

Combined liquid-liquid extraction and extractive distillation methods are operated as follows: the liquid hydrocanbon mixture from which the aromatic constituents should be separated is conveyed to a multi-stage extractor countercnrrent to the selective solvent.

A liquid phase (raflinate) is collected at the extractor end to which the solvent is fed, the said phase comprising non-aromatic hydrocarbons with a low solvent and aromatic hydrocarbon content, a phase (extract) being collected at the other end of the extraction system, with a high solvent and aromatic hydrocarbon and a low nonaromatic hydrocarbon content.

The extract is conveyed to an extractive distillation column at the top of which the non-aromatic products together with a certain quantity of aromatic products are collected. This mixture is recycled to the extraction system at the end of which the extracted phase is separated. Aromatic hydrocarbons are usually recovered from the solvent containing aromatic hydrocarbons and traces only of nonaromatic hydrocarbons, by distilling in a steam current through admission of live steam to the foot of the distillation column at the top of which the non-aromatic products together with a certain quantity of aromatic products are collected. This mixture is recycled to the extraction system at the end at which the extracted phase is separated. Aromatic hydrocarbons are method of separating aropure form from mixtures conusually recovered from the solvent containing aromatic hydrocarbons and traces only of non-aromatic hydrocarbons by distilling in a steam current through admission of live steam to the foot of the distillation column.

Various high-boiling solvents can be used for the purpose, such as =diethylene glycol, dipropylene glycol and sulpholene.

These solvents should meet the requirements. They should have a higher boiling point than the aromatic hydrocarbons to be extracted, they should be heat-stable and they should have satisfactory dissolving power and selectivity towards aromatic hydrocarbons. The selectivity can be varied by adding an antisolvent, such as water.

The temperature and pressure conditions currently employed for extraction vary within very wide limits from room temperature and atmospheric pressure to about 150 C. and a few atmospheres.

The above described processes are, however, objectionable under various aspects. For instance they are subject to a certain loss in aromatic hydrocarbon content in the refined phase. Moreover, the quantity of extract from the top of the extractive distillation column, which is recycled to the bottom of the extractor is very high and reaches in certain cases of the extracted hydrocarbons.

Thus, the extractive distillation step of the conventional method becomes expensive owing, to the large heat quantity to be supplied. Moreover repeated heating of the highboiling selective solvent is the cause of a certain decomposition of the solvent and consequently necessitates regeneration and purification of a part thereof at least. Also owing to such decomposition, products are formed which are more corrosive than the pure solvent, which entails hard wear on the apparatus employed and obstruction thereof by the resulting sludge.

By azeotropic distillation non-aromatic constituents can be removed by means of a stripping agent provided they do not exceed 12%. Also, it is only possible to separate non-aromatic constituents having a carbon atom number of which equals or is smaller than the carbon atom number of the aromatic hydrocarbon, from which separation is desired. Therefore, when aromatic hydrocarbons are to be separated the distillation of the individual aromatic hydrocarbons should be preceded by a distillation of a non-aromatic fraction by means of suitable stripping agents.

It has now been found in accordance with this invention that it is possible to obtain pure aromatic hydrocarbons with an exceptionally high yield while avoiding the abovementioned drawbacks.

According to the present invention we provide a method of separating aromatic hydrocarbons in a pure or substantially pure state from mixtures of liquid hydrocarbons containing them, comprising charging the mixture to be extracted to a multi-stage extractor at high temperature with a high boiling organic selective solvent, collecting from the said extractor a liquid extract phase with a high aromatic hydrocarbon content and a ramnate liquid phase essentially comprising nonaromatic hydrocarbons, conveying the extract after cooling to a decanter in which two distinct liquid phases are separated, recycling, the heavier liquid phase to the extractor preferably the upper portion of the extractor, conveying the lighter liquid phase to an azeotropic distillation column to which acetone vapor is admitted, and after fractionation, conveying as a reflux to the bottom of the extractor the lighter fraction of the non-aromatic ratfinate together with any aromatic hydrocarbons present in the raftinate.

The raifinate and overhead product of the azeotropic distillation column may be treated together with water in following fundamental order to separate an aqueous-acetonic phase from which acetone and a parafiinic phase containing any aromatic hydrocarbons present in the raffinate. The paraifinic phase is finally distilled in order to separate the lighter fraction which is recycled to the end of the extractor from which the extract is collected, thereby additionally recovering the aromatic hydrocarbons present in the raffinate. The acetone may be recovered from the aqueous-acetone phase and recycled.

The advantages of this procedure are obvious, for it avoids aromatic hydrocarbon losses in the ralfinate and repeated heating of the high-boiling selective solvent. The recycling of light paraflms prevents the presence of heavy parafiins at the extractor bottom, and thus paraffinic components can be removed by one distillation step employing acetone and pure aromatic hydrocarbons can be obtained.

This procedure avoids heavy refiuxes to the extractor which make extractive distillation by conventional methods expensive on account of the large heat quantity required, it further avoids part decomposition of the highboiling selective solvent, and finally provides a full recovery of all materials in the extraction process of aromatic hydrocarbons from the liquid hydrocarbon mixtures.

Among preferred solvents for carrying out this invention diethylene and dipropylene glycols, either alone or mixed together, and sulpholane may be mentioned.

In the preferred embodiment of the method the solvent contains 2% to 8% Water, which limits may, however, be expanded to 1% to 15% by weight. The extraction temperatures preferably range between 80 and 130 C., the pressure being maintained at values such that the extractor content is in a liquid phase.

The temperature to which the extracted phase is cooled is lower by 30% at least than the extraction temperature and preferably ranges between C. and 30 C.

The moist solvent-feed ratio can vary within fairly wide limits, namely 2:1 to :1. In the preferred embodiment of this invention, however, the ratio ranges between 4:1 and 8:1 by weight.

The apparatus employed for extraction is conventional, such as perforated tray columns, columns with filling bodies and rotary discs.

By this method aromatic hydrocarbons are very easily separated in a pure form by refining the extracted phase, thereby two liquid phases are formed, the solvent phase being recycled to the extractor, the other phase being azeotropically distilled with acetone.

In the preferred embodiment of the method the recycled solvent still having a portion of the hydrocarbons dissolved therein is fed at the second or third theoretical tray to the end of the extractor at which the raflinate is collected, the light paraflinic hydrocarbon flow being preferably fed at the second or third theoretical tray to the end of the extractor at which the extract is collected.

An embodiment of the invention will now be described with reference to the accompanying diagrammatic drawmg.

In a pilot plant a hydrogenated reforming petrol cut is fed to the extractor 1 through line 11 at a point situate about half-way the raflinate and extract extraction points.

The composition of the cut is as follows: aromatic constituents 69.3%, paraflin 30.7% by weight.

The aromatic components contain: benzene 40.8%, toluene 51.6%, and xylols 7.6% by weight, the pararfinic constituents containing 6.7 and 8 carbon atoms in the molecule.

The rate of feed of the mixture was 470 kg./ hr.

A diethylene glycol water mixture is fed through line 12 at a rate of 3,300 kg./hr. The water quantity in the mixture is 4.8%, feed being effected at the second theoretical tray calculated starting from the end at which the raffinate is collected.

When utilizing the recycled solvent the quantity fed through line 12 is 3,800 kg./hr. approximately.

About 420 kg./hr. light parafiins from the distillation column 10 are fed through line 13, the said parafiins containing, the small quantities of aromatic hydrocarbons present in the rafiinate. This feed is eifected at the second theoretical tray calculated starting from the extractor end at which the extract is collected.

About 370 kg./ hr. aromatic hydrocarbons from the top phase in the decanter 2 are fed through line 14.

These aromatic hydrocarbons contain about 10% by weight paraflinic hydrocarbons, feed being effected at the first theoretical tray calculated starting from the reactor end at which the extracted phase is collected.

The extractor 1 is a rotary disc column having about 10 theoretical trays. The temperature in the extractor is about C., the pressure being about 4.5 atmospheres.

4,580 kg./hr. approximately of extract are collected through line 15, the extract being cooled down to room temperature and conveyed to the decanter 2 which is maintained at the same pressure as the extractor 1. The solvent phase is drawn from the decanter through line 16 and is recycled to the extraction reactor 1, after having been preheated, at a rate of about 3,800 kg./hr. The lighter phase is drawn through line 17 at a rate of about 760 kg./ hr. and is charged in part to the extraction reactor 1 through line 14, the remaining part being fed at a rate of about 390 kg./hr. to the azeotropic distillation column 3 through line 18.

Acetone vapor is supplied to the foot of the azeotropic distillation column 3 through line 19 at a rate of about 68 kg./hr. About kg./hr. acetone paraffine mixture are collected at the top of the column through line 20. The tail product of column 3 is drawn at a rate of about 325 kg./hr. through line 21 and is fed to the columns 4, 5 and 6 for distillation of benzene, toluene and xylols, respectively. Thus, about 132 kg./hr. benzene, about 167 kg./ hr. toluene and about 26 kg./ hr. xylols are discharged through lines 22, 23 and 24, respectively.

The small solvent quantities at the foot of the distillation column 6 are recycled through line 25 to the extractor 1.

The rafiinate is discharged through line 26 from the extractor 1 and is conveyed together with the overhead product of the azeotropic distillation column 3 through line 27 at a rate of about 636 kg./hr. to the water washing column 7. About 30 kg./ hr. water are fed to the top of the washing column 7 through line 28, the product being discharged to the decanter 8 through line 29.

The aqueous acetonic phase is collected from the decanter 8 and is supplied through line 30 to the acetone distillation column 9.

The acetone is collected from the top of column 9 and is recycled through line 31 to the azeotropic distillation column after having been vaporized. Water is drawn from the foot of the column 9 and is conveyed through line 32 to the washing column 7.

Paraflins containing the aromatic constituents present in the rafiinate are supplied from the decanter through line 33 to the distillation column 10. Lighter parafiins are collected from the top of the column 10 together with small quantities of aromatic constituents and are fed to the extractor 1 through line 13. Heavy paraffins are drawn at the foot of the column 10 through line 34 at a rate of about kg./hr. The aromatic hydrocarbons which are extracted with yields of about 100% exceed 99% in purity.

What we claim is:

1. A method of separating aromatic hydrocarbons from mixtures of liquid hydrocarbons containing them comprising:

(1) charging the mixture to be separated to a multistage extractor at elevated temperature together with a high boiling selective solvent selected from the group consisting of diethylene glycol, dipropylene glycol, sulpholane and mixtures thereof,

(2) collecting from the extractor a liquid extract phase with high aromatic hydrocarbon content and a liquid raflinate phase essentially comprising non-aromatic hydrocarbons,

(3) cooling said extract and conveying it to a decanter,

(4) allowing two liquid phases to separate in the decanter,

(5) recycling the heavier solvent phase from the decanter to the extractor,

(6) conveying the lighter phase with high aromatic hydrocarbon content from the decanter to an azeotropic distillation column,

(7) azeotropically distilling said lighter phase with acetone, to obtain an overhead fraction and a bottoms fraction with a high aromatic hydrocarbon content,

(8) recycling said railinate together with the overhead product resulting from the azeotropic distillation to said extractor after fractionation and separation of acetone, and

(9) recovering said high aromatic hydrocarbon content fraction.

2. Method of separating aromatic hydrocarbons in a pure or substantially pure state from mixtures of liquid hydrocarbons containing them, comprising charging the mixture to be extracted to a multi-stage extractor at high temperature together with a high boiling organic selective solvent selected from the group consisting of diethylene glycol, dipropylene glycol, sulpholane and mixtures thereof, collecting from the said extractor a liquid extract phase with a high aromatic hydrocarbon content and a liquid raflinate phase essentially comprising nonaromatic hydrocarbons, conveying the extract after cooling to a decanter in which two distinct liquid phases are separated, recycling the heavier solvent phase to the extractor, conveying the lighter liquid phase to an azeotropic distillation column to which acetone vapor is admitted azeotropically distilling to recover a high aromatic content bottoms fraction, and after fractionation, conveying as a reflux to the bottom of the extractor the lighter fraction of the non-aromatic raflinate together with any aromatic hydrocarbons present in the ramnate.

3, Method as claimed in claim 2, in which diethylene glycol is employed as a solvent.

4. Method as claimed in claim 2, in which sulpholane is employed as a solvent.

5. Method as claimed in claim 2, in which the solvent employed contains 1% to 15% b.w. water.

6. Method as claimed in claim 2, in which dipropylene glycol is employed as a solvent.

7. Method as claimed in claim 2, in which the extraction temperatures range between and C.

8. Method as claimed in claim 2, in which the pressure in the reactor is higher than the atmospheric pressure, and is suflicient to maintain the extractor content in a liquid phase.

9. Method as claimed in claim 2, in which the weight ratio of solvent to mixture to be extracted ranges between 2:1 and 15:1.

10. Method as claimed in claim 9, in which said weight ratio is between 4:1 and 8:1.

11. Method as claimed in claim 2, in which the extracted phase is cooled to a temperature lower by 30% at least than the extraction temperature, to thereby effect separation into two liquid phases.

12. Method as claimed in claim 2, in which the extract phase in cooled to a temperature ranging between 10 and 50 C.

13. Method as claimed in claim 2, in which the heavier solvent phase from said decanter is fed to a second or third theoretical stage of the extractor at the end at which the raffinate is collected.

14. Method as claimed in claim 2, in which the lighter liquid phase from said decanter is fed in part to the first theoretical tray of the extractor at the end at which the extracted phase is withdrawn, the remaining portion being azeotropically distilled by means of acetone vapor.

15. Method as claimed in claim 2, in which a light paraffin flow is fed at the second or third theoretical tray to the end of the reactor at which the extract is collected.

16. Method as claimed in claim 2, in which the aromatic constituent content in the mixture to be extracted ranges between 30% and 80% by weight.

17. Method as claimed in claim 2, characterized by the fact that any olefin content in the mixture to be extracted is lower than 0.1% by weight.

References Cited UNITED STATES PATENTS 2,153,116 4/1939 Eichwald 208-311 2,567,172 9/1951 Arnold et al. 208-313 2,773,006 12/ 1956 Carver et al. 208-311 2,999,892 9/1961 Papadopoulos et al. 260-674 HERBERT LEVINE, Primary Examiner.

US. Cl. X.R. 208-3ll, 321, 333; 260-674 

